1. Field of the Invention
The present invention relates generally to downhole reservoir characterization and in particular to a method and apparatus for real time identification of gases diffused out of a formation fluid sample. Formation fluid samples are obtained and gases are allowed to diffuse from these fluid samples through a semi-permeable membrane into an evacuated chamber. The gases are analyzed in the evacuated chamber by a mass spectrometer or residual gas analyzer (RGA) and a processor which identifies and distinguishes between gases such as C1–C5, H2S, C02, N2 and other gases or vapors present in a downhole reservoir fluid sample.
2. Summary of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled into the earth by rotating a drill bit attached at to the end of a drill string. Modern directional drilling systems generally employ a drill string having a bottom hole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or by rotating the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible regarding the nature of the hydrocarbon formation, in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems and wire line analysis applications, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested with other test equipment such as wire line tools, which are used to further analyze and monitor the formation.
One type of post-drilling test involves producing fluid from the reservoir and collecting such fluid samples downhole in tanks for transport to surface laboratories where Pressure-Volume-Temperature (PVT) studies and fluid properties such as density, viscosity and composition are measured. Also, one can measure the downhole fluid pressure at several depths and, from this pressure gradient, calculate the fluid's density.
Fluid samples extracted downhole are typically analyzed weeks to months later in a surface laboratory to identify and quantify gases present in the fluid. It is time consuming to retrieve fluid samples downhole and send them to a surface lab for analysis of gas content Moreover, surface analysis requires removal of the fluid sample and the tool from the borehole for testing the sample before additional exploration and/or production activities occur. Thus, there is a need for a real-time downhole method and apparatus for detection, distinction and quantification of gases in the formation.